In breast surgery and anatomy, the composition of the breast itself could vary from patient to patient due to respective proportions of glandular and fatty tissue. There is no easy and reliable way to predict these proportions. The purpose of this study was to evaluate the respective proportions of glands and fat in breasts. By analysing mastectomy specimens we were able to quantify the amounts of glandular and fatty tissue in the resected breasts. Twenty one breasts were studied following mastectomy for pure in situ carcinomas. The total volume and weight of the specimens were recorded and completed by specific densities of glandular and fatty tissue for each breast. This group was submitted to a calculation of proportions of glands and fat. The percentage of fat volume in the total breast volume varied from 7 to 56% and the percentage of fat weight in the total breast weight varied from 3.6 to 37.6%. This great variability in the respective proportions of fat and glands in the evaluated specimens was not significantly correlated to age and body mass index.